Weather and Climate 天氣及氣候 What is the difference between weather and climate?  Weather refers to the conditions of the atmosphere over a short period of time at any one place.  Climate refers to the average conditions of the atmosphere over a long period of time (usually more than 30 years) at any one place.

Elements of Climate TemperaturePressureWindPrecipitation

Air Temperature Heat balance of the Earth  The sun is the main source of heat for the Earth  After insolation reaches the Earth, heat is released and transferred from the Earth’s surface through radiation, conduction, convection and advection

Direction of prevailing wind Latitude Distance from the sea Ocean Current Altitude Aspect Cloud cover Nature of the ground Length of the day Tempearture is affected by

Latitude High Low - small angle of incidence of insolation - insolation has to pass through a thicker layer of clouds - lower intensity of sunlight - larger of incidence of i angle nsolation - insolation has to pass through a thinner layer of clouds - higher intensity of sunlight Lower temperature Higher temperature

Altitude High Low The lower density of air and dust particles traps less heat The higher density of air and dust particles traps more heat Lower temperature Higher temperature Altitude

Temperature inversion - temperature increases with increasing altitude - it occurs when warm wind blows over cold wind - it also occurs when there is a rapid loss of heat from the Earth’s surface due to radiation at night

Aspect  Refers to the direction of a slope faces  Sun-facing slopes receive more insolation than shaded slopes  Effect of aspect on temperature is obvious in temperature areas

Nature of surface Nature of surface Land Sea bare forested Greatest diurnal and annual range of temperature lower diurnal and annual range of temperature than bare ground Lowest diurnal and annual range of temperature as sea heats up and cools down slower than land

Cloud cover Thick Thin -less sunlight can reach the ground surface in the day time -more long-wave radiation can be trapped at night -More sunlight can reach the ground surface in the day time -Less long-wave radiation can be trapped at night Small diurnal range of temperature Greater diurnal range of temperature

Distance from the sea Coastal Inland Sea heats up and cools down slower than land Land heats up and cools down faster than sea On-shore wind lowers the temperature in summer and raises its temperature in winter Temperature cannot be regulated by on-shore wind Small annual range of temperature Extreme range of temperature Distance from the sea

Ocean current Warm Cold Temperature is raised in the presence of on-shore wind Temperature is lowered in the presence of on-shore wind Ocean currents

Ocean Currents They move along the coast. Warm ocean currents: move towards the poles Cold ocean currents: move towards the equator Onshore winds blowing over a warm current, bring a warming effect in the coastal areas in winter. e.g. North Atlantic Drift Onshore winds blowing over a cold current, bring a cooling effect to the coastal areas in summer.

e.g.the coast of Norway (North Atlantic Drift) the west coast of Canada (North Pacific Drift) W Europe (Oporto): E Northern American (New York): winter T higher winter T lower Effects of warm ocean currents: When warm ocean currents move from lower latitudes to higher latitudes, warmth is carried from equatorial/tropical regions.

This maritime moderating effect is particularly significant if the winds blow onshore. This results in a distinct difference between the winter temperatures on the western and eastern margins of continents on similar latitudes.

Effects of cold ocean currents When the winds blow onshore, cold currents tend to lower the temperatures of those coastal areas. The cold Labrador Current brings a colder winter temperature: -same latitude (38 degree N) -Washington D.C. (0.5 degree C) Lisbon (9.5 degree C)

Effects of cool ocean currents Cool ocean currents exert cooling influence on summer temperatures in coastal areas of continents. The winds blow onshore during summer, and coastal temperatures are lowered. e.g. The cool Californian Current tends to lower the summer temperatures San Diego 19.4 degree C 35.5 deg N e.g. The warm Gulf Stream higher the summer temperatures Charleston 27.2 degree C 38.4 deg N

Direction of prevailing wind Onshore wind 向岸風 Offshore wind 離岸風 Moderating effect: warm in summer and cool in winter in coastal areas Extreme climate: hot in summer and cold in winter in the affected areas Direction of prevailing wind

Length of the day Long e.g. along the Equator Short e.g. in higher latitudes -receive more insolation -Hotter -receive less insolation -Colder

World distribution of temperature Isotherms do not form stationary straight lines parallel to lines of latitude

Causes of variations in isotherms 1. Different air temperature over land and sea surfaces 2. Different height of the land surface 3.The apparent “movement” of the overhead sun between the summer and winter seasons, which causes a slight northward or southward “movement” of isotherms.

World distribution of temperature Temperature pattern in January

Temperature pattern in July

Pattern of annual range of temperature